The present invention relates to a prestressed construction element of composite structure.
The invention also concerns a method for the fabrication of the prestressed element.
Being entailed with the use of materials of different elastic properties as well as of differing values of Young's modulus and yield characteristics, building technology has been forced to rely on special constructions. In use of these constructions, part of the materials are imparted to such conditions within the core of the element that under load not even the weakest element of the construction is imposed to bending or deformations leading to cracking. In other words, the aim is towards a construction of homogeneous behavior. Together with certain strength and economical prerequisites, these requirements lead to an ever increasing use of prestressed structures. These structural elements are comprised of steel structures embedded in concrete. Prior art prestressed constructions are slab or massive beam constructions fabricated by, e.g., slipform-casting concrete around pretensioned steel wires or cables. Prestressing is implemented by tensioning the wires or cables between two anchoring points. The distance between the anchoring points is typically 50 . . . 100 m. After a sufficient hardening of the concrete, the elements are cut to desired length. As disclosed in FI publication print 54638 and DE publication print 2 035 385, also known are non-prestressed support structures of beam-like form, which are castable into concrete.
A disadvantage of prior art technology is that concrete structures are continuously deformed under stress up to a certain limit. In the art this behavior is called creep. Creep is described to be primarily dependent on two different factors: diffusion of excess water in a fresh, hardened concrete and its removal from the structure, and plastic deformation of the amorphous component of concrete. The magnitude of creep depends on the amount of cement used and its degree of hydration, mixing water volume used in concrete fabrication, as well as the quantity, quality and shape of stone aggregate, the quantity of entrapped air and the distribution of pores in the concrete mix, that is, factors which are difficult to determine beforehand in a systematic and exact manner. By contrast, creep is also related to the magnitude of prestressing as well as the span of the construction and other loads imposed on the structure. Whilst prestressed structures are aimed to achieve a predetermined magnitude of curvature or desired straightness, the magnitude of prestressing, creep, and other factors lead to an uncertainty in reaching a desired final state of conditions. For instance, presently such constructions as, e.g., hollow-core slabs require the use of floor levelling compounds whose thickness in the worst places, which are associated with the bending of the slabs caused by prestressing, may be even up to 2 . . . 5 cm, while in the thinnest places, only a few millimeters. Levelling of hard concrete at the building site, together with all indirect costs such as material, worktime, idle time, heating, interest on bound capital, and other costs, leads to excess costs which according to very conservative estimates exceed 50% of the initial fabrication costs of the slab. In addition, present prestressed constructions of hollow-core slabs are extremely critical as to their fire resistance characteristics. In the state of the art technology, pretensioned cables can be designed into a limited number of places, and their thickness choices are also limited. Due to the fabrication method, all pretensioning tendons in a hollow-core slab are parallel and, consequently, so far no prestressed transversal reinforcements have been used.
Further, non-prestressed support structures lack the advantages of prestressed structures, and prior art constructions have been primarily developed to ease installation and transport of operations for small building constructors.